Lecture 24: Genetic disorders

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Questions and Answers

What is the genotype of an unaffected individual in the context of Huntington's disease?

hh (correct)

HhHh

What is the characteristic of the mutant alleles in Huntington's disease?

Less than 7 trinucleotide repeats

More than 40 trinucleotide repeats (correct)

Exactly 35 trinucleotide repeats

No trinucleotide repeats

Why does the abnormal HTT allele persist in human populations?

Because it provides a selective advantage

Because it is a recessive disorder

Because affected individuals cannot have children

Because patients may have children before diagnosis (correct)

What is the age range during which symptoms of Huntington's disease typically manifest?

30-45 years of age Signup and view all the answers

What is the prevalence of Huntington's disease in Western countries?

5-7 people per 100,000 Signup and view all the answers

What is the genotype of a parent if a child has a 50% chance of inheriting Huntington's disease?

Hh or HH Signup and view all the answers

What is unique about the incidence of Huntington's disease in the Lake Maracaibo region of Venezuela?

It is the highest in the world Signup and view all the answers

How many trinucleotide repeats are normally found in the HTT gene?

7-35 Signup and view all the answers

What is the major symptom of sickle cell disease?

Severe pain from obstruction of blood flow Signup and view all the answers

Why do heterozygous individuals have greater resistance to malaria?

Malaria infected red blood cells tend to sickle and are removed by macrophages Signup and view all the answers

What is the purpose of a pedigree?

To indicate the presence or absence of a trait for each family member Signup and view all the answers

What is the genotype of Anne, if her son has sickle cell disease?

Heterozygous Signup and view all the answers

What is the relationship between sickle cell disease and malaria?

Heterozygous individuals have a selective advantage in areas where malaria is prevalent Signup and view all the answers

What is the characteristic of red blood cells in individuals with sickle cell disease?

They are prone to sickling Signup and view all the answers

What is the majority of our cells' chromosome composition?

Diploid Signup and view all the answers

Which chromosomes do not determine our sex?

Chromosomes 1-22 Signup and view all the answers

What is the term for the 23rd pair of chromosomes in humans?

Sex chromosomes Signup and view all the answers

What is responsible for the initiation of male sex determination in humans?

SRY gene Signup and view all the answers

What is the genotype of an affected male in an X-linked dominant disorder?

XAY Signup and view all the answers

What is the term for the chromosomes that are inherited from our mother and father?

Maternal and paternal chromosomes Signup and view all the answers

What is the term for the process by which an organism's sex is determined?

Sex determination Signup and view all the answers

What is the characteristic of X and Y chromosomes in humans?

Non-homologous Signup and view all the answers

What is the probability that a child will inherit an X-linked recessive disorder if the mother is a carrier and the father is unaffected?

0.50 Signup and view all the answers

What type of genotype would an affected male have in an X-linked recessive disorder?

Hemizygous (XaY) Signup and view all the answers

What is the characteristic of an X-linked recessive disorder?

One abnormal allele is sufficient to cause the disorder Signup and view all the answers

What is the genotype of a carrier female in an X-linked recessive disorder?

Heterozygous (XAxa) Signup and view all the answers

What is the result of a mutation in the OPN1LW or OPN1MW gene?

Insensitivity to green light Signup and view all the answers

What is the main reason for the clotting deficiency in Haemophilia A and Haemophilia B?

Abnormal alleles code for proteins that cannot participate effectively in blood clotting process Signup and view all the answers

What is the specific clotting factor deficient in Haemophilia B?

Clotting factor IX Signup and view all the answers

What is the typical phenotype of a female carrier of Haemophilia A or Haemophilia B?

Generally healthy phenotype Signup and view all the answers

What is the epigenetic mechanism that influences the diversity in phenotypes for female carriers of Haemophilia A or Haemophilia B?

X chromosome inactivation (lyonisation) Signup and view all the answers

What is the mode of inheritance of the disorder shown in the given pedigree?

X-linked recessive Signup and view all the answers

What is the significance of X chromosome inactivation (lyonisation) in females?

It prevents females from having twice as many X chromosome gene products as males Signup and view all the answers

Study Notes

Sickle Cell Disease

Sickle cell disease is a recessive phenotype that affects millions of people worldwide.
It causes severe pain from obstructed blood flow.
Heterozygous individuals (sickle cell trait) have greater resistance to malaria.
Malaria-infected red blood cells tend to sickle and are removed by macrophages.

Pedigrees

A pedigree is a family tree indicating the presence or absence of a trait for each member.
Conventions: for unaffected male, for unaffected female, for affected male, and for affected female.

Huntington's Disease

Huntington's disease is an autosomal dominant disorder.
Healthy individuals are homozygous recessive.
Affected individuals are homozygous dominant or heterozygous.
The abnormal HTT allele is characterized by additional CAG repeats in the sequence.
Symptoms manifest between 30-45 years of age.
Patients may have had children before diagnosis.

Autosomes and Sex Chromosomes

Autosomes are chromosomes 1-22, which don't determine our sex.
X and Y chromosomes are the sex chromosomes, which determine our sex.

Sex Determination

Sex in humans is determined by X and Y chromosomes.
The sex-determining region on the Y chromosome (SRY gene) is responsible for the initiation of male sex determination.

Sex-Linked Disorders

X-linked dominant disorders:
- Affected females can have a homozygous dominant or heterozygous genotype.
- Affected males are hemizygous (one abnormal allele is sufficient to cause the disorder).
X-linked recessive disorders:
- Affected females have a homozygous recessive genotype.
- Affected males are hemizygous.
Y-linked disorders:
- The abnormal allele exists on the Y chromosome and can only be passed from father to son.

Red-Green Colour Blindness

Red-green colour blindness is an X-linked recessive disorder.
It results from mutations in the OPN1LW or OPN1MW gene, causing abnormal opsin pigments in cone cells of the retina.
Affected individuals have trouble distinguishing between some shades of red, yellow, and green.

Haemophilia A and Haemophilia B

Haemophilia A and Haemophilia B are X-linked recessive disorders.
They are characterized by clotting deficiencies due to abnormal alleles coding for proteins that cannot participate effectively in blood clotting.
Haemophilia A is the most common type, with a deficiency of clotting factor VIII.
Haemophilia B (Christmas disease, Royal disease) has a deficiency of clotting factor IX.

Haemophilia Manifestation in Female Carriers

Female carriers have a heterozygous genotype and generally have a healthy phenotype.
Some may have mild symptoms of haemophilia, and severe symptoms are rare but possible.
X chromosome inactivation (lyonisation) influences the diversity in phenotypes for female carriers.

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Description

This quiz explores the genetic aspects of sickle cell disease, its symptoms, and why the sickle cell allele persists in human populations.